You’ve got a mobile radio and separation kit. You’re ready to begin installation. Now comes the most important question—where do I mount the antenna?
Sometimes I think it’s a conspiracy among automakers. Every year, it gets more challenging to find a good mounting location on a vehicle. But with a little imagination and planning, you can get maximum performance from a mobile antenna.
Where Do I Put It?
On a vehicle, antennas can be installed in several locations: the roof, fenders, trunk, and window glass. A high mounting position is better for performance, along with having as much metal mass under the antenna as possible. Another important thing to consider is keeping the antenna clear of roof pillars and hatch doors when you can.
Specific locations considered best for VHF/UHF antennas are (in order of preference): roof center, center of the trunk lid, fenders, trunk/hood grooves, and on-glass mounting. This ranking is determined by the amount of ground plane provided by the position and clearance from obstructions. As you consider locations, also determine where your feedline will enter the vehicle. You don’t want to have a large run lying on the car body or mash it in a door frame.
If you’ve seen police, fire, and commercial vehicles, you’ll notice their antennas are mounted directly to the body, improving the ground connection. This is the preferred method, but some of us get squeamish about drilling holes in the sheet metal, especially if it’s a leased vehicle. For many, direct mounting isn’t an option. But there are several other antenna mounting alternatives that will provide good performance.
Antenna Mounts
One of the simplest and easy to install mounts is the magnet mount. It’s convenient, removable, but is generally intended as a temporary antenna, though many hams leave them on indefinitely. It’s grounded through capacitive coupling between that magnet and the metal under it. This grounding method is adequate for VHF/UHF frequencies, and I’ve found these to be a good solution to getting the antenna on the center of the roof without having to drill a hole. The downside is they can collect crud on the bottom of the magnet and scratch paint if carelessly removed.
Mirror and roof rack mounts are other alternatives that are easy to install. However, mirror brackets and roof racks aren’t always electrically grounded to the body. If you choose this route, you should consider a ½ wave no ground plane antenna (NGP).
Trunk lip/universal lip mounts are popular because they’re easy to place on most vehicles. They’ll fit on the edge of a trunk, hood, door or the rear hatch on an SUV. A popular bracket, the Diamond K400, will pivot so the antenna can be adjusted into an upright position. Once placed, lip mounts attach with set screws which are tightened into the bottom edge of the metal. To make a ground connection, you will need to scrape away paint directly under the set screws to expose bare metal. A little dielectric grease on the exposed metal will help prevent corrosion.
One way to assure a reasonably good ground is to use fender/bracket mounting. Comet No-Hole Fender Brackets use existing fender bolts on trucks and SUVs to support the antenna and provide a ground. Angle brackets can be attached with stainless sheet metal screws inside doors, rear hatches, and trunk groove, but check to make sure the gap is large enough to easily open and close without striking the bracket.
Glass mount antennas come as a kit with mount and a ½ wave antenna. They’re designed for tempered automobile glass with a nominal thickness of 5/32″. Due to changes in glass designs in newer cars, there are some restrictions as to where they mount. They can’t be installed over embedded AM/FM antennas, on areas with aftermarket tinting film, or on glass with metallic content, such as solar coat or solar-cool.
Choosing Your Antenna
Antennas and mounts are usually sold separately. This is a good thing because you can mix and match according to your needs. Be sure that the antenna base matches your mount. For example, an NMO mount requires an NMO antenna. Other types of mounts include UHF, N, and 3/8″ threaded stud.
There are three basic types of mobile antennas: 1/4 wave, 1/2 wave, 5/8 wave, etc. Each has its own characteristics. For example, the signal radiating from a 1/4 wave antenna is directed at higher angles, making it ideal in urban environments. The design of a 5/8 wave antenna is designed to direct the signal more toward the horizon, ideal for flat terrain where signal coverage is sparse. Half-wave antennas are utilized when there is a poor or nonexistent ground, such as vehicles with bodies made of fiberglass/composite materials.
Size matters. If you park in a garage and use an antenna that extends over the roof line, you’ll want to consider a short antenna or one with a fold-over feature to avoid scraping the overhead door frame. A trunk lid or fender mounting lets you use up to a 40″ antenna, which will give you more radiating efficiency.
Look for high antenna gain figures, but take them with a grain of salt. Antenna gain is a relative measurement, and values can be misleading if the reference point isn’t disclosed. Antenna manufacturers will often state gain figures based on dBi or dBd, which is referenced to an isotropic (dipole) radiator. Be sure you’re not comparing apples to oranges.
